This invention relates to an exhaust gas purifying device for an internal combustion engine, in which a three-way catalyst is disposed in the exhaust system of the engine to operate most effectively for purifying the engine exhaust gases by removing toxic components such as CO, HC and NOx contained in the engine exhaust gases. More particularly, this invention relates to an exhaust gas purifying device of the kind above described in which feedback control is applied to the air-fuel ratio at the intake or exhaust side of the engine on the basis of the detected concentration of oxygen in the engine exhaust gases, so that the air-fuel ratio at the intake or exhaust side of the engine can be controlled to permit most effective operation of the three-way catalyst for the purpose of purification of the engine exhaust gases.
An intake control system and an exhaust control system for an internal combustion engine are known in the art. The known intake control system includes a three-way catalyst disposed in the exhaust system of the engine together with an oxygen sensor detecting the concentration of oxygen in the engine exhaust gases, so that the amount of fuel and/or the amount of air at the intake side of the engine can be controlled on the basis of the detected value of the oxygen concentration in the engine exhaust gases. The known exhaust control system controls the air-fuel ratio at the exhaust side of the engine by supplying a suitable amount of secondary air to the exhaust side of the engine (at a point upstream of the three-way catalyst) on the basis of the detected value of the oxygen concentration.
In order that the three-way catalyst can most effectively operate in these control systems, the air-fuel ratio must be controlled to lie within a narrow region W in FIG. 1 which shows the relation between the air-fuel ratio of engine exhaust gases and the purification percentages of CO, HC and NOx in the engine exhaust gases. This air-fuel ratio region W extends over a narrow width on opposite sides of the stoichiometric air-fuel ratio of the air-fuel mixture (which ratio is set at about 14.5 in the present application), and it is a common practice to employ a sensor sensing the stoichiometric air-fuel ratio of engine exhaust gases and to apply feedback control on the basis of the output of the sensor to make the air-fuel ratio of the engine exhaust gases equal to the stoichiometric one.
Known sensors of this kind include an oxygen sensor utilizing an oxygen ion conductive metal oxide such as zirconium oxide whose output voltage varies in response to the concentration of oxygen in engine exhaust gases, and an oxygen sensor utilizing a metal oxide semiconductor such as titanium oxide whose electrical resistance value varies in response to the concentration of oxygen in engine exhaust gases.
FIG. 3 shows, by way of example, the general arrangement of an exhaust control system in block diagram. The exhaust control system shown in FIG. 3 includes an air pump 2, a solenoid operated valve 3, a control circuit 4, an oxygen sensor 5, and a three-way catalyst 7 for controlling the air-fuel ratio of exhaust gases in an exhaust manifold 22 of an internal combustion engine 1. Such an exhaust control system was used in an experiment conducted by the inventors, with the air pump 2 replaced by factory air, and an additional oxygen sensor 6 disposed downstream of the three-way catalyst 7.
The carburetor is set to provide an air-fuel mixture having an air-fuel ratio of, for example, 13 or 14 which is richer than that of the ideal or stoichiometric air-fuel ratio of 14.5, and the secondary air is supplied in on-off fashion to the engine exhaust gases discharged as a result of combustion of the air-fuel mixture having such a rich air-fuel ratio. The air-fuel ratio of the engine exhaust gases is 13 when no secondary air is supplied to the engine exhaust gases and the carburetor is set to provide the air-fuel mixture having the air-fuel ratio of 13, while the air-fuel ratio of the engine exhaust gases is 16 when the amount of secondary air supplied to the engine exhaust gases is selected to turn the rich air-fuel ratio of 13 into a lean air-fuel ratio of 16. Thus, the rich air-fuel ratio of 13 and the lean air-fuel ratio of 16 are repeatedly provided by the on-off of secondary air supplied to the engine exhaust gases. Therefore, the mean value of the air-fuel ratio of the engine gases is (13+16)/2=14.5 which corresponds to the effective operating region of the three-way catalyst, so that the three-way catalyst can most effectively purify the engine exhaust gases. The oxygen sensor 5 is used as the means for the on-off supply of the secondary air. The oxygen sensor 5 has such an operating characteristic that its electromotive force changes abruptly between 0 and 1 volts with the variation of the air-fuel ratio of engine exhaust gases between its rich and lean levels, and this operating characteristic of the oxygen sensor 5 is utilized to actuate the solenoid operated value 3 by the output signal of the oxygen sensor 5 applied through the control circuit 4 thereby attaining the on-off of the secondary air supplied to the engine exhaust gases.
The curve (A) shown in FIG. 2 represents the output of the oxygen sensor 5 in the exhaust control system shown in FIG. 3 when a conventional sensor including a sensing element of zirconium oxide and not having any special catalyst around the sensing element was used as this oxygen sensor 5. It will be seen from FIG. 2 that the electromotive force generated by this oxygen sensor 5 shows a sharp variation in the lean air-fuel ratio range in which the air-fuel ratio is relatively larger than the optimum air-fuel ratio (which is approximately equal to the stoichiometric air-fuel ratio) at which the three-way catalyst 7 operates most effectively. Therefore, in the air-fuel ratio feedback system utilizing such an oxygen sensor, the three-way catalyst cannot operate fully effectively.
As one of means for solving such a problem, the inventors proposed to dispose a metallic catalyst such as platinum (Pt) at a point upstream of the oxygen sensor 5 or around the sensing element of the sensor 5. It was proved that, according to this method, the partial pressure of oxygen in the engine exhaust gases was equilibrated by virtue of the catalytic action of the metallic catalyst, and as a consequence, the electromotive force generated by the oxygen sensor 5 varies only in the vicinity of the optimum air-fuel ratio for the three-way catalyst as shown by the curve (B) in FIG. 2. In this case too, however, the value of the air-fuel ratio did not lie within the desired air-fuel ratio region W shown in FIG. 1 in spite of the purification by the three-way catalyst and deviated toward the right-hand side or leaner side of the desired air-fuel ratio region W. That is, the result of purification was such that the purification percentage of NOx was so low although the purification percentages for CO and HC were satisfactory. In other words, the purification performance of the three-way catalyst was such that the mean air-fuel ratio of the engine exhaust gases obtained as the result of feedback control on the basis of the output of the oxygen sensor was, as it were, controlled to lie on the leaner side of the desired air-fuel ratio region W in FIG. 1.
It will thus be seen that the three-way catalyst cannot still fully effectively operate even when a catalyst, such as, a metallic catalyst, platinum (Pt), different from the three-way catalyst (Pt-Rh) is disposed upstream of the oxygen sensor or around the sensing element of the oxygen sensor.